In a transistor having an MIS (Metal Insulator Semiconductor) structure, there is a problem in which an impurity contained in a gate electrode diffuses into a gate insulating film and a semiconductor substrate to generate a characteristic fluctuation of the transistor.
For example, to suppress diffusion of the impurity, there is a technique of providing a diffusion prevention layer for the impurity between the gate electrode and the gate insulating film. However, for example, charges may be trapped by trapping levels in the diffusion prevention layer to cause an additional characteristic fluctuation of the transistor.